Data output impedance control

ABSTRACT

The present invention provides an apparatus and method for monitoring the functioning of a special operational mode on an integrated circuit module without the need for a special or dedicated pin. By monitoring the data output pins of the module operation in a special operational mode and premature interruption thereof, is detected. Delayed transition from a state of low impedance to a state of high impedance during the data output cycle is indicative of the special operational mode. The modules which usually have tri-state devices on their output lines are provided with delay circuitry to delay the transition of the tri-state device, during the data output cycle, from a state of low impedance to a state of high impedance while the device remains in a special operating mode.

This application is a division of application Ser. No. 07/895,971, filedJun. 9, 1992, now U.S. Pat. No. 5,455,517.

TECHNICAL FIELD

This invention relates generally to the field of integrated circuitsand, more particularly, to a method and apparatus for assuring themaintenance of an operational mode on an integrated circuit module,e.g., burn-in or other test modes on a memory module.

BACKGROUND ART

Integrated circuit semiconductor devices or modules since their firstfabrication and use in the 1960's have truly revolutionized our world.Commonly referred to as integrated circuit chips or computer chips,their development and sophistication has accelerated steadily over theyears. The first chips had only a few circuit elements; nowadays, chipsor modules commonly have tens of thousands of circuit elements on themand can perform a wide variety of sophisticated functions.

One aspect of this increasing sophistication involves the addition ofcircuitry which allows the module to enter various special operationalmodes on receipt of the appropriate sequence of signals. Test modesimplemented by circuitry built into the module constitute one broadcategory of operational modes. The signals used to put the module into aspecial operational mode generally consist of the standard signals themodule normally uses during most functions, with the normal sequence ofsignals altered. This avoids the need for special dedicated pins orconnections. However, glitches, such as power interruptions, or failureto maintain the proper sequence of signals can cause the module to exitthe special operational mode with no indication that it no longerremains in the desired operational mode.

In some instances, the electronics industry has standards, promulgatedby various professional organizations, to follow in fabricating moduleswith standard operational modes. The Solid State Products EngineeringCounsel, JEDEC, on or about Apr. 22, 1987 approved LTRB JC-42.3-86-95A"Optional Special Modes For Address Multiplexed DRAM." This JEDECpublication is incorporated herein by reference. The standards set forthin the JEDEC publication define a scheme for: a.) controlling variousspecial modes for address multiplexed DRAMS; b.) the logic required toenter, control and exit from the special modes; and c.) the basicspecial test modes plus other special test modes and operational modes.One problem not resolved by the standards is, how to monitor the moduleto determine that the module is in a special operating mode and toassure that the module remains in the special operating mode for adesired period of time and does not prematurely exit from the specialoperating mode.

An example will serve to illustrate the problem. One type of test modecircuitry built into a multiplexed DRAM, when activated, increases thevoltage in the memory cells above the normal voltage levels the cellsexperience during normal operation. Burn-in tests conducted shortlyafter manufacture of the memory module would use this feature. A burn-intest consists of: a.) elevating the voltage in the memory cells of theDRAM to a potential of at least 1 or 2 volts above the normal voltagethe DRAM would experience during normal operation; b.) placing the DRAMin an ambient temperature higher than it would normally experience andc.) maintaining these conditions for a specified period of time. Specialtest ovens with appropriate equipment to put the chips or modules intothe necessary test mode can conduct mass tests on large numbers ofmodules at one time.

Burn-in tests stress the oxide or dielectric connections of a DRAM. Theoxide or dielectric connections of a DRAM are located at the gate of thetransistors, at the connection between the storage capacitor and thetransistor, between the transistor and the bit lines, as well as atother locations throughout the DRAM. Burn-in when properly done, helpseliminate DRAMS with defective oxide or dielectric connections. Thesedefective connections can be a major source of failure for DRAMS duringnormal operation. Consequently, when properly conducted, burn-in testsincrease the reliability of the end product.

The JEDEC standards provide for the initiation of a test mode on a DRAMthrough a reversal of the sequence of the row address signal (RAS),column address signal (CAS) and read/write signal (W), coupled withdecoding of a portion of the row address. However, there is no provisionfor monitoring module operation in a test mode. Further, as known in theart, the DRAM can exit from the test mode prematurely for severalreasons including a momentary power glitch or an improper refresh cycle,both of which would not affect the normal operation of the DRAM and arenot otherwise noticeable. If such a failure occurs during a burn-in testand the DRAM falls out of its test mode, given the current state of thetechnology, the test equipment does not sense this, nor does the testerreceive any warning thereof. When the test fails, the memory devicestested experience a higher than usual failure rate during normaloperations.

One approach to solve this problem would be to add an extra pin orconnection to the DRAM solely for the monitoring of special operationalmodes. However, this uses a valuable connection or pin which couldotherwise be used for other purposes. The challenge then is to provide amethod and apparatus for monitoring test and other special operationalmodes and for signalling a loss or change in such an operational mode,without the necessity of adding a special or dedicated pin or connectionto the module.

DISCLOSURE OF INVENTION

The present invention provides a method for monitoring the maintenanceof a special operational mode on an integrated circuit module to assureits proper functioning, using one or more existing data out pins. Thisfeature avoids the need for special or dedicated connections or pins onthe module. The method of the invention has the further advantage of notinterfering or inhibiting the normal function of the pin(s) used tomonitor the operational mode.

The present invention also provides an efficient, simple and spacesaving apparatus, easily fabricated and integrated into the existingstructure of the module to implement the method of the invention.

In accordance with the principles of the present invention, anintegrated circuit module having an input, internal circuitry receivingsignals from the input and an output receiving signals from the internalcircuitry and disabled a first predetermined time delay after theinternal circuitry has completed operating in a first operating mode, isprovided with a circuit for indicating that the internal circuitry isoperating in a second operating mode. This circuit consists of a firstmeans coupled to the input for detecting that the internal circuitry isbeing operated in the second operating mode and second means coupled tothe first means and to the output for disabling the output a secondpredetermined time delay after the internal circuitry has completedoperating in the second operational mode.

In a further aspect, a third means coupled to the first means isprovided for detecting premature interruption of operation in the secondoperating mode and, in response thereto, reverting to disabling theoutput after the first predetermined time delay.

In a further aspect of the invention, an output control circuit formonitoring an operational mode of the integrated circuit module isprovided. The output control circuit includes a tri-state device with aninput connection for the reception of data, an output connection fortransmission of data and a disable control responsive to a disablesignal for changing the tri-state device from a state of low impedancewhich allows the transmission of data to a state of high impedance whichprevents the transmission of data. Additionally, a disable circuit whichproduces a disable signal on completion of said operational mode isprovided with means for switching the disable signal between a firstline and a second line. The first and second lines are connected to thedisable control of the tri-state device. The first line allows for animmediate transmission of the disable signal to the tri-state device andthe second line has a delay means for delaying the transmission of thedisable signal to the tri-state device. A detection means for detectinginitiation of the operational mode, and in response thereto, for settingthe switching means to connect the disable signal to the second line isalso provided. Finally, monitoring means for monitoring the operation ofthe integrated circuit module and upon detection of a prematureinterruption of said operational mode for setting the switching means toconnect the disable signal to the first line is also provided.

A single such output control device can provide different delays fordifferent operational or test modes. In a multiple output module, eachoutput line can be provided with an output control device of the presentinvention.

In accordance with yet another aspect of this invention, there isprovided a method of determining whether an integrated circuit modulehaving an output control circuit is in a test mode. The output controlcircuit transitions from a low impedance state to a high impedance stateafter a first time period during normal operation of the module. Themethod includes the steps of maintaining a low impedance state of theoutput control circuit for a second time period greater than said firsttime period when the module is operated in a test mode, and samplingdata output of the module at a time falling between an end of the firsttime period and an end of the second time period. The method may furthercomprise the step of reverting to switching from the low impedance stateto the high impedance state after said first time period if the testmode is prematurely interrupted.

In a further aspect of this invention, a method for detecting a changein a special operational mode of an integrated circuit module whichoutputs data on an output line is provided. This method includes thesteps of detecting the initiation of a special operational mode of themodule; monitoring for a premature interruption of the specialoperational mode of the module; and modifying the flow of data on theoutput line in response to a premature interruption of the specialoperational mode. The modifying step may comprise halting the flow ofdata by transitioning from a state of low impedance to a state of highimpedance on the output line. The method may further comprise the stepsof providing a disable signal; delaying said disable signal by a timedelay; halting the flow of data on said output line in response to saiddelayed disable signal and reducing said time delay in response to amonitored premature interruption of said special operational mode.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other aspects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings in which:

FIG. 1 is a schematic overview of an integrated circuit memory modulewhich includes the apparatus of the present invention;

FIG. 2 depicts a prior art output control circuit;

FIG. 3 depicts an output control circuit constructed in accordance withthe principles of the present invention;

FIG. 4 depicts an alternate version of the output control circuit of thepresent invention;

FIG. 5 presents a timing diagram showing a sequence of signals fornormal operation of a memory module during both the read and writecycles; and

FIG. 6 presents a timing diagram depicting a sequence of signals to setthe integrated circuit memory module into a test mode and the differencein the output signal caused by the present invention during the readcycle.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention provides a scheme for monitoring a specialoperational mode of an integrated circuit module. Although the presentinvention can be fabricated on and used to monitor operational modes ona broad spectrum of integrated circuit modules, the preferred embodimentdescribed herein details use of the invention on a semiconductor memorydevice, specifically a DRAM, for monitoring test modes. Semiconductormemory devices, their fabrication and methods of testing are well-knownin the art. Most integrated circuit memory devices or DRAMS includeinternal testing circuitry. The testing circuitry built into the memorydevice or DRAM provides a variety of options including the ability toelevate the voltage levels in the memory cells for such test as burn-in,or the testing of time delays. Such tests and the circuitry to implementthem are well known in the art. Initial testing using the test circuitryfabricated on the DRAM usually occurs shortly after manufacture.

FIG. 1 depicts schematically an integrated circuit memory module 20 withits salient features and their relation to the data output impedancecontrol of the present invention, four examples of which are shown at21A, 21B, 21C and 21D. The output impedance control devices 21A-21D,provide a means for individually controlling the output of tri-statedevices 22A, 22B, 23C and 23D, respectively. Tri-state devices, theirstructure, and function are well-known in the art. One version is knownas a TRI-STATE GATE™ (registered trademark of National SemiconductorCorporation). When fabricated as part of the output connections of amodule, tri-state devices provide a convenient switching means whichallows the output of an integrated circuit module to be turned off andon as desired.

The tri-state devices 22A-22D receive data over lines 46A-46D,respectively, from data bus 43. Data bus 43 in turn connects to memoryarray 23. When the individual tri-state devices 22A-22D are turned on,i.e., put into a state of low impedance, by an appropriate controlsignal, data transmits to output lines 47A-47D from the associatedtri-state device. Correspondingly, when the tri-state devices 22A-22Dare shut off, i.e., put into a state of high impedance, by anappropriate signal, data ceases to flow through them onto lines 47A-47D.The output impedance control devices 21A-21D of the present invention,described in detail below, provide a means for delaying the transitionfrom a state of low impedance to a state of high impedance of eachtri-state device separately. This delay in transition of each individualtri-state device is monitorable on lines 47A-47D. The activation of theoutput control devices 21A-21D and their functioning depends on themaintenance of a special operational mode as explained in detail below.

The memory array 23 receives data over a standard input data bus 42. Astandard address bus 44 connects to both the memory array 23 and a testmode decoder 33. An output enable G line 59, a row address select (RAS)line 58, a column address select (CAS) line 57 and a read/write (W) line56 all connect to the memory array 23 as well as to the test modedecoder 33. The normal functions of all these lines are well-known tothose skilled in the art. The test mode decoder 33, a familiar device inthe art, implements the special operational circuitry pursuant to theJEDEC standards previously cited herein.

Only those features of a memory device germane to the present discussionappear in FIG. 1. Various devices and circuitry schematically depictedin FIG. 1 provide a way for putting the module into various operationalmodes to test various aspects of the module. Voltage generator 32 isused for tests such as burn-in to raise the voltage levels of the memorycells of memory array 23 above the normal maximum level. Circuitry 31can change the timing of various signals to test maximum and minimumoperating margins.

The module 20 enters a test mode when the appropriate signals, to bedescribed below, are received by the test mode decoder 33. The test modedecoder 33 activates when the appropriate sequence of signalspromulgated by the JEDEC standards enter it over lines 58, 57 and 56.Upon receipt of the proper sequence of signals, test mode decoder 33sets mode latches 24A-24N over line 55 with the mode set signal MSET.The test mode decoder 33 also decodes a row address or addresses itreceives over the address bus 44 and accordingly activates a specificmode latch or latches, as the case may be, over one or more of controllines 48A-48N. The number of mode latches and associated control linescan vary as symbolized by the nth mode latch 24N. The mode latches areused to activate various operational modes including those referred toin the JEDEC standards previously referred to and incorporated herein.

For example, assume the first mode latch 24A activates upon receivingthe MSET signal over line 55 and the appropriate signal over line 48Afrom test mode decoder 33. In turn, the mode latch 24A activates thevoltage generator 32 over bus 41. The voltage generator 32 might set thevoltages in the memory cells at a slightly higher than normal operatingpotential to prepare the memory array 23 for a burn-in test.Simultaneously, mode latch 24A activates by appropriate electricalconnections along bus 41 one of the output control circuits, e.g., 21Aof the present invention. Circuit 21A then would delay the output of thetri-state device 22A resulting in a change in the output signalcharacteristics over line 47A which is monitored outside the module 20.Likewise, other mode latch detectors such as 24B, 24C or 24D, couldactivate other tests such as timing tests with circuitry depicted at 31.The mode latches, when activated, in turn activate corresponding outputcontrol circuits.

If for some reason a mode latch 24A receives a reset signal MRESET overline 54 or the mode latch resets due to a momentary power loss or forsome other reason, the special operational mode or test may prematurelyterminate. Upon this occurrence, the corresponding output controlcircuit 21A receives over bus 41 the same signal which causedtermination of the operational mode. This would result in a change onthe output line 47A, monitorable outside the module, and recognizable asa premature transition from a state of low impedance to high impedanceat the end of the data output cycle.

FIG. 2 illustrates a prior art output device of a memory module. Asdepicted, a tri-state device 22 has an input along line 46 from dataline 43 and an output along line 47. Tri-state devices, well-known inthe art, output data when in a state of low impedance and when switchedto a state of high impedance do not allow the passage of data, thusshutting the line off like a switch. Line 61 provides an output disablesignal which controls the transition of the tri-state device 22 betweena state of high impedance, when the transmission of data is preventedand a state of low impedance during which data can be transmittedthrough the tri-state device 22.

Referring back to FIG. 1, as is well-known in the art, the row addressselect signal RAS on line 58, the column address select signal CAS online 57, or some combination of the two traveling over bus 41 and outthrough line 61 act as the output disable signal which effectively causethe tri-state device 22 to transition from a state of high impedance toa state of low impedance or visa versa at the appropriate time.

The output impedance control circuit of the present invention will nowbe described in detail. FIG. 3 depicts one version of an outputimpedance control circuit 21 of the present invention. Circuit 21includes a first line 65 connected to a disable control 19 of tri-statedevice 22, and a second line 17 also connected to disable control 19.Second line 17 includes a delay circuit 64. A switch 63 is provided forselectively switching a disable signal on line 61 between first line 65and second line 17. Switch setting is controlled by a control signalprovided over line 62. In operation, an output disable signal comingover line 61 from bus 41 encounters switch 63 in its path. When switch63 is connected to line 65 the tri-state device 22 operates in itsnormal fashion. In response to an appropriate signal indicatingoperation in a special operational mode, received over line 62, switch63 connects the disable signal line 61 to second line 17 containing thedelay circuit 64. Connection through delay circuit 64 delays thetransmission of the disable signal and, thus, delays the transition oftri-state device 22 from a state of low impedance to a state of highimpedance. The delay in transition would accordingly be monitored atoutput line 47 by appropriate equipment well-known in the art. Delayselect line 62, which controls switch 63 connects through bus 41 to theappropriate mode latch 24 and is controlled by a signal received fromthat mode latch. As long as the mode latch 24 is set and has theappropriate test mode implemented, then switch 63 will connect to thedelay circuit 64. If the mode latch 24 resets and the operational modeor test condition terminates, switch 63 will then connect to line 65 andthe equipment monitoring line 47 will sense the change as a result of alack of an expected delay in the transition from low impedance to highimpedance during the data output cycle.

Output impedance control circuit 21 may comprise multiple delay lines asshown in FIG. 4. The version depicted in FIG. 4 provides a three-wayswitch 66 which allows for switching of the output disable line 61between the normal operation line 65, a first delay circuit 64A oralternatively a second delay circuit 64B. This arrangement would providealternative delay circuits with different timing characteristics asneeded. The setting of switch 66 is controlled by signals received overlines 62A and 62B.

Delay circuits 64A and 64B can provide different time delays, e.g.,associated with different operational modes. The internal circuitry ofdelay circuit 64A could consist of a series connection of monostabledevices, e.g., inverters. In such case, the delay circuit would providea specific time delay. A bi-stable device or latch 64B provides anotherappropriate delay means for a delay circuit. Once switch 66 puts thedelay circuit 64B containing the latch into the circuit by connectingthe output disable signal line 61 to a set line 67, of the latch 64B,until a reset input 68 of the latch receives an appropriate RESETsignal, the output disable signal will not reach the tri-state device22. Any number of different signals sent over bus 41 to reset line 68could provide a signal to reset the latch and thus allow transmission ofthe output disable signal to the tri-state device 22. The signal whichcommences the next operating cycle of the memory device provides onepotential source of such a reset signal. Of course, output impedancecontrol device 21 may include additional lines containing delaycircuits.

The operation of a memory module incorporating output impedance controldevices of the present invention will now be described with reference tothe timing diagrams of FIGS. 5 and 6.

FIG. 5 shows the normal sequence of operating signals encountered duringa normal read and write operation of a module. The row address selectsignal RAS transits to a low state before the column address CAS or theread/write signal W does. Such signaling does not activate the test modedecoder 33 of FIG. 1. Consequently, the data out line remains in a low Zor low impedance state for the normal time period, time A, as depictedon the data out line in FIG. 5.

FIG. 6 depicts the sequence of signals used to activate the test modedecoder and ultimately the test circuitry according to the JEDECstandards. Typically, the column address select signal CAS and theread/write signal W go low before the row address select signal RAS goeslow. Test mode decoder 33, FIG. 1, then activates and transmits the modeset signal MSET to the mode latches over line 55. Additionally, testmode decoder 33 decodes the row address number received over address bus44 and activates the appropriate mode latch. As long as the mode latchactivated does not reset by receiving the reset signal MRESET over line54, or the mode latch is not reset for some other reason, the impedancecharacteristics of the output signal monitored on output line 47 duringthe read cycle appears as shown by the data out signal 79 in FIG. 6. Ineffect, a delay in the transition to high impedance, by a time B, isadded to the data out signal during the read cycle. Thus, during time B,the data output signal remains in a state of low impedance. Appropriatetesting equipment or other equipment monitoring the data out line 47 andstrobing for the condition of the output signal during time B wouldaccordingly sense a low impedance state. By contrast, if the mode latchhad reset and accordingly the operational mode or test mode prematurelyterminated, and the switch setting in circuit 21 reverted to the firstline, the equipment strobing at line 47 would detect a high impedancestate when strobing during time B, as depicted on data out signal 78,FIG. 6. This then would signal that the operational mode or test modehas been lost.

Several operational mode latches can be activated and function on themodule 20, FIG. 1, at one time. The test mode decoder 33 when sendingthe MSET signal over line 55 would also receive several addresses overaddress bus 44 which it would decode and then accordingly activate theappropriate mode latches, e.g., 24A-24D over the appropriate controllines 48A-48D. The activated mode latches would in turn activate theappropriate circuitry for the operational modes selected. Additionally,the mode latches activated also activate the appropriate outputimpedance control circuits, e.g., 21A-21D, over lines 41. Thus, thestatus of each mode latch and the operational mode it initiates can bemonitored on corresponding output lines 47A-47D.

In the system described above, the signals generated by the mode latches24 to implement the various operational modes also control the outputimpedance control circuits 21. Thus, the output impedance controlcircuits react to a change in the signal directly from the mode latchesand not from monitoring an internal state of the circuitry implementingthe operational mode or an overall condition or state of the module 20.Alternatively, special compare or monitoring circuitry 29, FIG. 1, builtinto the module 20 during manufacture could provide a direct means formonitoring the actual condition on the module 20. Such circuitry knownin the art could directly monitor various conditions on the module 20including the voltage levels in the memory cells or other operationalmodes.

Such compare circuitry, as indicated by circuitry 29, would be activatedby the test mode decoder 33 in the same manner as the mode latches. Thecircuitry would then generate an appropriate signal which it wouldtransmit over bus 41 to the appropriate circuit 21, and activate thatoutput impedance control circuit. For example, such activation wouldconnect switch 63 to delay circuit 64 of FIG. 3. The compare circuit 29would then monitor the actual state of the operational mode or someother condition on the module 20. If it detects a change, it would thensend the appropriate switch reversion signal to circuit 21. Thus, theactual condition of the module would directly control the switch 63 and,accordingly, whether or not the expected delay in the transition fromlow impedance to high impedance on line 47 would appear while strobingfor such during the data output cycle.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

We claim:
 1. A method of determining whether an integrated circuitmodule having an output control circuit is in a test mode, the outputcontrol circuit transitioning from a low impedance state to a highimpedance state after a first time period during normal operation of themodule, comprising the steps of:maintaining a low impedance state of theoutput control circuit for a second time period greater than said firsttime period when the module is operated in a test mode; and samplingdata output of the module at a time falling between an end of the firsttime period and an end of the second time period to determine whetherthe output control circuit is in the low impedance state, therebydetermining whether the module is in the test mode.
 2. The method ofclaim 1 further comprising the step of reverting to switching from thelow impedance state to the high impedance state after said first timeperiod if the test mode is prematurely interrupted.